HBD Update

1. HBD Update Itzhak Tserruya DC meeting, December 12, 2007

2. Reminder from last month • HBD assembly started in August 6 and only 4 stacks were successfully assembled till October 20. Since then all (at least 5) attempts to assemble triple GEM stacks failed. • The plan we adopted at the HBD meeting on November 6 was to pursue two main venues in parallel: • a) Find the origin of the problem and fix it: • Ilia to BNL • b) Modify the operating conditions and/or the detector design so as to operate the GEMs as they are: • Try a different gas mixture like CF4/Ar • 4 GEM stack operation • Very good progress was achieved over the last four weeks in both fronts

3. Looking for the problem (I) • Ilia spent the last 4 weeks at BNL. The goal was to demonstrate that there are no miracles or mysteries across the ocean and that he can reproduce here the results routinely obtained at the Weizmann Institute on triple GEM stacks. • I’ll spare you the details of the stupid problems that occurred several times and were related to the setting-up of a new working system and to equipment failures. • The test procedure was as close as possible to the one used at the Weizmann: same test box, same divider, same preamp, same CAEN power supplies, assembly of GEMs inside the test box in a clean room (he used the “clean room” at Instrumentation, which turned out to be not so clean but was nevertheless good enough), gas system flushing CF4 through the test box….

4. Looking for the problem (II) • Four triple GEM stacks were assembled and tested with the following results (in chronological order): • “Thanksgiving” stack (3 standard GEMs that Ilia brought from Weizmann) was successfully operated at a gain of 5000 during the long Thanksgiving week-end. • “Hanuka” stack (3 standard GEMs from SUNY that were successfully used in a 4 GEM stack test) was also successfully operated at a gain of 5000. • A 3rd stack was assembled made of standard GEMs that were ranked as bad in the first inspection at SUNY: bottom and middle GEMs were found normal however the top GEM showed a non-ohmic behavior. Washing with de-ionized water did not cure the problem. • The “Test” stack, made of a new Au plated GEM (w/o CsI) and two standard GEMs from the “Hanuka” stack was assembled and successfully tested to a gain of 5000. • The goal is take these same GEMs and assemble them inside the glove box at SB and reproduce the results obtained at BNL. Then evaporate CsI on the top GEM and repeat the test. This will tell us whether or not CsI is at the origin of the instabilities.

5. Fe55 x-ray spectrum • Measured yesterday on the triple GEM “test” stack showing a gain of 5700 at a total voltage of 3700V i.e VGEM = 497 V.

6. In the meantime conditions have improved considerably at SB • A particle meter installed inside the glove box revealed several pockets of dust concentration: • The principal enemy seems to be “dead air” • Places where the flow from the HEPA filter does not readily reach. • For example, when the Transport box lid is raised, the clean air is directed toward the ceiling and lower regions see much more dust. • The second enemy are horizontal surfaces. • When there is no activity the ambient dust is class 50. However, large effects are seen when “blowing on the floor” (class 1000-2000) , “blowing on the lids”, or “blowing in the gain test box” (class 10,000). • One of the CO2 cartridges (used for removing dust) was tested extensively and found to actually be a source of dust. No more CO2 cartridges are used inside the glove box. Instead a N2 pressure line with an in-line 0.5 micron filter is used to blow gas from a bottle. • An ULPA vacuum cleaner was purchased at SB to routinely vacuum clean the glove box. The ambient gas is typically now at class 20 and a “blow test” of the gain test box produces only minor increases in the dust level (50  400) as compared to the previous blow test (50  10000).

7. Changing the operating scheme: 4 GEM stacks • Two 4-GEM stacks were successfully operated inside the glove box at SB: • - some trips in the beginning • - ran 5000 gain from the very beginning. • The 4-GEM stack seems to be a stable alternative. However the implementation of this solution will require more work, more GEMs, will affect the hadron blindness of the detector and will require redesign of the mesh electrode. • The 4-GEM stack is our fall back solution if we do not manage to solve the triple stack stability problem.

8. Changing the operating conditions (I): CF4/Ar mixtures Adam C - BNL ~ 20V Benefit appears marginal: a 90/10 CF4/Ar mixture lowers the operating voltage by only ~20 V.

9. Changing the operating conditions (II): adding CsI to the middle GEM? The CsI layer on the top GEM yielded a gain increase by almost a factor of 2 in last year’s GEMs.. Is it due to photon feedback? If so the benefit should be highly enhanced if there is also CsI on the middle GEM. CsI stack wapping (SB) • The pink and blue dots show the gain from the same quad stack with the top two GEMs swapped (moving the CsI away from the very top). • The gain is the same essentially verifying that the gain effect is NOT photon feedback.

10. Changing the operating conditions (III): asymmetric operation? • COMPASS obtained a large improvement in stability by a non-uniform gain distribution across the three GEMs. They typically operate at V = 1V, 0.9V and 0.8V across the top, middle and bottom GEMs respectively. • However, COMPASS is limited by space charge effects, whereas we seem to be limited by quality. • We will nevertheless study whether there is anything to gain in stability with a non uniform gain distribution between the three GEMs a la COMPASS. Such a scheme, having the highest gain on the top GEM, will also increase hadron blindness .

11. Summary • We have made very good progress. • We are very close to isolate the origin of the problem. • We do have a fall back solution if we are not able to overcome the problem.